The disclosure relates generally to a capsule to be used in high pressure, high temperature applications. The disclosure relates generally to a capsule to be used with a high pressure apparatus. More particularly, the disclosure relates to a capsule used in conjunction with a high-pressure apparatus for processing materials in a supercritical fluid. Supercritical fluids may be used to process a wide variety of materials. Examples of supercritical fluids applications include extractions in supercritical carbon dioxide, the growth of quartz crystals in supercritical water, and the synthesis of a variety of nitrides in supercritical ammonia.
Processes that employ supercritical fluids are commonly performed at high pressure and high temperature within a pressure vessel. Most conventional pressure vessels not only provide a source of mechanical support for the pressure applied to reactant materials and supercritical fluids, but also serve as a container for the supercritical fluid and material being processed. The processing limitations for such pressure vessels are typically limited to a maximum temperature in the range between about 400° C. and 600° C. and a maximum pressure in the range between about 0.1 GigaPascals (also referred as “GPa”) and 0.5 GPa.
Processing material with supercritical fluids often requires a container or capsule that is substantially both chemically inert and impermeable to the solvent and any gases that might be generated by the process. The capsule should also be substantially impermeable to any gases or materials on the outside of the capsule. These capsules are commonly made in the form of cylinders, possessing a wall and two ends disposed opposite each other along the axis of the cylinder. In one approach, the material to be processed, along with a solvent (liquid) that forms a supercritical fluid at elevated temperatures, is introduced into a capsule at low temperature. After the capsule has been sealed and returned to near room temperature, the capsule will possess an elevated internal pressure as dictated by the vapor pressure and temperature of the solvent (liquid) within the capsule. In the case of ammonia at room temperature, the pressure within the capsule is approximately 150 pounds per square inch. This internal pressure can cause deformation, strain, cracks, leaks, and failure of the capsule, particularly for capsules larger than several inches in dimension, and/or when the capsule is fabricated from a soft metal such as silver or gold.
Some legacy approaches use a capsule, principally of fused silica or quartz or glass, placed in contact with a pressure medium, which is within an outer capsule. This capsule within a capsule is designed to use the pressure medium between the two capsules to either counterbalance the pressure from the inner capsule or to provide an overpressure so that the inner capsule is under a compressive or neutral stress, rather than under tension, since the materials of construction of the inner capsule (fused silica, quartz or other glass) tend to fail when under tension. The outer capsule, therefore, is the principle pressure vessel and must be able to withstand the longitudinal and radial stresses as dictated by the fluid and its temperature inside. Depending on the capsule material and the intended volumetric capacity of the capsule, this may require very thick capsule walls and ends, which may be limiting, especially on a manufacturing scale and for materials such as silver, gold, or platinum. Other approaches use reinforcement members attached to the ends of the capsule. Such reinforcement members attached to the ends of the capsule often do not prevent failure of the capsule due to radial forces on the body (e.g., walls) of the capsule, and they do not reinforce against longitudinal forces that act to elongate the capsule, both of which are significant for capsules of large dimension.
From the above, it is seen that improved techniques for processing materials in a high pressure apparatus are highly desirable.